Circuit breaker overload sensing device



June 27, 1967 ET GRYcTKo 3,328,639

CIRCUIT BREAKER OVERLOAD SENSING DEVICE June 27. 1967 c. E. GRYc'rKoCIRCUIT BREAKER OVERLOAD SENSING DEVICE 2 Sheets-Sheet 2 Filed July l1965 um E United States Patent O 3,328,639 CIRCUIT BREAKER OVERLUADSENSING DEVCE Carl E. Gryctko, Haddon Heights, NJ., assigner to l-T-ECircuit Breaker Company, Philadelphia, Pa., a corporation ofPennsylvania Filed July 1, 1963, Ser. No. 292,050 16 Claims. (Cl.S17-4l) My invention relates to a circuit breaker overload sensingdevice, and more particularly to a solid-state circuit arrangementoperable in conjunction with the heater element of a conventionalcircuit breaker to electrically trip the breaker in a manner permittingincreased adjustability and reliability of operation. The solid-statecircuitry arrangement may also operate in conjunction with theconventional instantaneous load sensor to electrically trip the circuitbreaker responsive to either a predetermined time delayed overloadcondition or an instantaneous short circuit condition.

lt is well known in the circuit breaker art to trip the breaker upon theoccurrence of a moderate overload condition existing for a prolongedduration, with such operation usually being provided by acircuit-connected heater element arranged to deflect a bimetallicelement responsive to the thermal condition resulting from Inoderateoverload. The deflection of the bimetallic element is calibrated toprovide the desired tripping time characteristic according to the timeduration of the particular overload condition. The deflection of suchbimetallic element is then mechanically interconnected to a latcharrangement for operating the circuit breaker contacts responsive topredetermined detlection of the bimetallic element. Exactness ofoperation of such an arrangement requires close calibration of Vthebimetal deflection and accurate alignment of the various mechanicalmembers interconnected between the thermally responsive element and triplatch. Accordingly, variations in alignment between such individu-alelements adversely affects the calibration of the unit. Further, themagnetic field generated within the closely contined operating membersof typical present-day circuit breakers aects the alignment anddistortion of the various members, thereby varying the calibrationaccuracy of the tripping characteristic; with such atects being greatestfor high current applications.

My invention advantageously avoids these problems by electricallytripping the circuit breaker responsive to the existence of apredetermined thermal condition, as generated by the heater element of aconventional circuit breaker unit. More specically, I provide a circuitarrangement including .a thermally responsive transducer, such as athermistor, in heat transfer relationship with the circuit breakerheater element. Although the ensuing discussion will be directed to theuse of a thermistor, it is to be understood that other thermallyresponsive transdncer means, such as a thermocouple, may likewise beemployed.

An electrical characteristic of the thermistor element, such asresistance, varies responsive to the overload thermal condition of theheater element. The thermistor element is in circuit relationship with asoli-d state switch, such as a uni-junction transistor, to transfer theunijunction transistor between its blocking and conducting states,corresponding to a predetermined resistance variation of the thermistorelement.

As a particularly advantageous aspect of my invention, the thermistorelement is placed within a voltage divider network to apply a biasingsignal to the emitter of the uni-junction transistor element. One of theelements of the voltage divider network is adjustable' to providevariation in the input biasing signal to the uni-junction transistorswitch element corresponding to predetermined 3,328,639 Patented June27, 1967 resistance variation of the thermistor element. Accordingly,selective adjustment of such an element, which for example may be aconventional potentiometer, permits a variati-on in the relationshipbetween the .thermally induced resistance change -of the thermistorelement and the biasing signal magnitude presented to the uni-junctiontransistor. Accordingly, such selective adjustment provides calibrationof the overload trip sensing circuitry in a simplified and reliablemanner, as contrasted to the various mechanical adjustments required inthe mechanical trip arrangements of the prior art.

The output ofthe uni-junction transistor may in turn be connected to thegate input terminal of .another switching element, such as a siliconcontrolled rectifier, for switching said rectiiier between its blockingand conducting states responsive to the aforesaid resistance variationof the thermistor elements.

The operation of the circuit breaker is preferably provide-d by aconventional type of under voltage trip unit of the general type shownin copending U.S. patent application Ser. No. 185,328 tiled Apr. 5,1962, entitled, Shunt Trip and Under Voltage Device, in the name ofFelix E. Myers and assigned to the assignee of the instant invention andnow United States Patent No. 3,175,064. The under voltage tripping relayis placed in parallel across the silicon controlled rectified device,such that when the silicon controlled rectifier is in its blockingcondition the voltage applied across the under voltage relay willmaintain the circuit breaker in its non-tripped position. Switching ofthe silicon controlled rectifier to its conducting state places a lowimpedance short across the under voltage coil, thereby causing thebreaker to trip. Alternatively, operation of the circuit breaker may beprovided by a shunt trip, saturable reactor, permanent magnet or otherwell known devices responsive to operation of the trip sensing circuitryof the instant invention.

As another advantageous aspect of my invention, a further switchingarrangement is provided to be operable in conjunction with aconventional short circuit or instantaneous load sensing means tosimilarly switch another uni-junction transistor between its blockingand conducting states responsive to a predetermined instantaneous loadcondition. The output signals of the overload and instantaneous loadsensing circuitry are presented to the gate input of the siliconcontrolled rectifier via isolating circuitry, thereby providingoperation thereof responsive to either time-delayed thermally inducedresistance variation of the thermistor element, or instantaneous loadconditions above a predetermined magnitude.

It is therefore seen that the basic concept of my invention resides inelectrically tripping a circuit breaker mechanism by virtue of asolid-state circuit arrangement operatively responsive to loadconditions sensed by conventional circuit breaker means.

It is therefore a prima-ry object of my invention to provide an improvedcircuit breaker overload sensing device of increased adjustability andreliability of operation.

A further object of my invention is to provide a circuity breakeroverload sensing device having a thermally responsive means in heattransfer relationship with a circuit breaker heater element, and circuitarranged to switch a solid state switching means between its blockingand conducting states.

Another object of my invention is to provide such an overload sensingdevice having an adjustable electrical component in circuit arrangementwith the thermally responsive means and solid state switching means topermit selective calibration of the operation thereof.

An additional object of my invention is to provide overload tripping ofa circuit breaker in a manner -avoiding a aaneen@ bimetallicinterconnection between the overload heater element and the circuitbreaker tripping mechanism.

Still a further object of my invention is to provide an electronic tripunit for a circuit breaker comprising solid state switching meanselectrically operative responsive to predetermined overload andinstantaneous current conditions to activate a relay trip means of aconventional circuit breaker.

Those as well as other objects of my invention will readily becomeapparent upon a consideration of the following drawings in which:

FIGURE l is -a block diagram illustrating the basic operation of myinvention in a form having both time-delayed thermal trip andinstantaneous short circuit trip portions.

y FIGURE 2 is a schematic ldiagram of an electrical trip arrangement inaccordance with FIGURE l.

FIGURE `3 graphically shows the operating characteristics of a typicalthermistor element.

Referring to the figures, thermal overload sensor 100 and instantaneouscurrent sensor 200 are connected intermediate line terminals 10, `20 andloa-d terminals 15, 25, in the conventional manner. Sensor unit 100typically includes a circuit connected heater element 102, and sensorunit 200 a circuit connected current transformer 202. As will besubsequently shown, the existence of predetermined load conditions beingpresented to the electrical circuitry of units 100 `or 200 provideselectrical output signals 150, 250l respectively. These signals are thenpresented to solid state switching circuit 300 via circuitry 400, thelatter insuring independent operation of solid state switch 300responsive to the generation of either signals 150 or 250. Solid stateswitch 300 in its conducting state presents a low impedance shortcircuit to the conventional circuit breaker trip coil 500, to therebyprovide tripping of the circuit breaker contacts 502. As is well known,heater element 102 will be at a temperature corresponding to the timeduration of overload conditions; and current transformer 202 will havean output corresponding to the instantaneous load condition, withrectifiers 206, 208 and loading resistors 210, 212 being provided toper-mit operation of the instantaneous current sensor circuitry 200responsive to either .a positive or negative fault current.

The source voltage for operation of sensing units 100, 200 and switchingcircuit 300 is provided by power supply 600. The input 23 of powersupply transformer 22 is preferably connected to line terminals 10, 20,with diodes 26, 28 being connected to the secondary output 24 for f-ullwave operation. Capacitors 32, 34 and resistor 30 form a low powerfilter arrangement to reduce the ripple of the power supply, with Zenerdiode 36 being provided to ensure a constant well regulated D.C.potential for oper-ation of the electrical sensing circuitry of theinstant invention. The energization of transformer primary 23 ispreferably shown controlled by Ian auxiliary switch 503 associated withthe circuit breaker, which closes :as the circuit breaker closes, toapply the operating potential to the electrical circuitry 100, 200, 300of my invention, with such operating potential being interrupted upontripping of the circuit breaker contacts 502. Switch 503 may be closedmanually or automatically in conjunction with the operation of circuitbreaker contacts 502 to de-energize transformer primary 23 upon opening4of circuit breaker contacts 502, with switch 503 being closed toenergize power supply `600 before circuit breaker contacts 502 may beclosed. It is naturally understood that a separate D.C. source couldalternatively 4be provided; however, the instant arrangement ofobtaining the operating voltage directly from the line sourceadvantageously avoids the requirement for such an auxiliary supply.

Referring to the operation `of the thermal overload sensor 100, athermistor element 104 is shown in heat transfer relationship withrespect to heater element 102,

it being understood that other thermally responsive transducer means,such as a thermocouple, m-ay alternatively be employed. The electricalcharacteristics of thermistor element 104, such as resistance, will varyin accordance with the temperature applied thereto, in the mannertypically shown in FIGURE 3. Thermistor element 104, in conjunction withvariable resistor 106 and fixed resistor 108, comprises a voltagedivider biasing network, the output signal of which is applied acrosscapacitor 117 to the emitter 12-6 of uni-junction transistor switchingelement 125. Rectifier 116 is preferably inserted along the seriescircuit Varrangement of elements 104, 106 and 108 to match thetemperature variations in the characteristics of the uni-junctiontransistor 125, to thereby provide increased uniformity of operationover temperature extremes. Resistors 112 and 114 provided in thermaloverload sensor stage 100 act as loading and biasing resistors for theuni-junction transistor element 125 of that stage.

Accordingly, the variation in the resistance of thermistor element 104will vary the magnitude of biasing signal 110 applied to thermistor 125.By proper adjustment of the voltage divider network in conjunction withthe characteristics of uni-junction transistor member 125, switchingbetween its blocking and conducting states may be effected correspondingto a predetermined temperature- `induced resistance variation ofthermistor element 104,

with such temperature being selectively adjustable by varying theadjustment of variable resistor 106. As, for example, consider the casewhere uni-junction transistor 12S will be transferred to its forwardconducting state corresponding to an applied potential of thirteenvoltsV at emitter 125 and will be in its blocking state corresponding toa lesser potential.

The voltage applied to the emitter 126 is given by th relationshipConsider the case where:

V126 :Vin X Vin=D.C. voltage applied to the voltage divider network (20volts) R106=2K ohms pot R108=5.2K ohms At a thermistor temperature of90, corresponding to normal operating conditions, its resistance will be4K (point A); thereby provi-ding 10 volts at emitter 126.

At a thermistor temperature of about C., corresponding to thepredetermined tripping point of the circuit breaker, the resistance ofthermistor 104 will `be 2K (point B), thereby providing 13 volts atemitter 126, causing it to transfer to its forward conducting state.

It is` naturally seen that suitable adjustment of variable resistor 106will provide a desired operating point suitably selected in accordancewith desired tripping times in relation to the thermal response ofmembers 102, 104 ,and the characteristics of uni-junction transistor125.

The single phase sensing arrangement shown in FIG- URE 2 may bemodified, as shown dotted, to operate as a three phase unit by theaddition of the similar arrangement of members 104', 106', 108 and 116to operate in conjunction with single uni-junction transistor storagecapacitor 117, to forward bias emitter 126 re sponsive to said signalexceeding ,a predetermined magnitude and transfer uni-junctiontransistor to its conducting state.

Considering now the operation of the silicon controlled rectifierswitching circuit 300, the plate terminal 327 is connected to the D C.potential source via loading resistor 332; the cathode terminal 328 isdirectly connected to the opposite terminal of the potential source; andgate terminal 326 is returned to the source via resistor 336. Diode 334is connected across resistor 336 to prevent the gate voltage from goingnegative. Resistors 332, 336 are selected in relation with theenergizing voltage applied to the solid state switch '300, such thatsilicon controlled rectifier 325 will be in its blocking conditioncorresponding to a zero gate signal. The application of a gating impulsecorresponding to t-he conduction of uni-junction switching members 125(or, as will be subsequently discussed7 225) permits discharge ofstorage capacitor 117 (or 220) to apply such a gating signal 150 (or250) respectively, to thereby switch silicon controlled rectifier 325 toits conducting state.

In its conducting state silicon controlled rectifier 325 is essentiallya low impedance short across under voltage coil 500 [of the typegenerally shown in the aforementioned U.S. patent application Ser. No.185,328], such that the circuit bre-aker unit -will be immediatelytripped. Accordingly, upon conduction of uni-junction transistor member125, a signal 150 is then applied to the gate terminal 326 of siliconcontrolled rectifier 325, causing it to abruptly switch from itsblocking to its conducting state. The conduction of silicon controlledrectifier 325 will short under voltage coil 500, thereby causing itsassociated circuit breaker contacts 502 to trip. Alternatively,operation of the circuit breaker may be provided b-y a shunt trip,saturable reactor, permanent magnet or other well known devices actuatedresponsive to operation of the trip sensing circuitry of the instantinvention, with appropriate modification of the circuitry arrangement ofFIGURE 2 being provided.

Reference is now made to the instantaneous current sensor unit 200having uni-junction transistor switching means 225 operable responsiveto short circuit conditions; in a manner similar to that above-describedto conjunction with uni-junction transistor element 125. Uni-junctiontransistor 225 is switched by the input signal arrangement of adjustableresistor 214, connected to the center tap 203 of current transformer202, with the opposite phased output terminals of current transformer202 providing an output signal to the emitter terminal 226.Alternatively, other transducer means, such as a Hall effect generator,may be employed for instantaneous current sensing. Resistors 232, 234are the combined loading and biasing resistors of stage 200. Rectifier216, in series with adjustable resistor 214, operates in a mannersimilar to rectifier 116 of stage 100km compensate for temperaturevariations of uni-'junction transistor element 225. Rectifier 218,intermedi-ate the common junction 219 and the emitter 226 ofuni-junction transistor 225, is inserted to prevent the instantaneouspulse from influencing the charge on capacitor 220. The circuitcomponents are so adjusted such that a magnitude of signal 204corresponding to an instantaneous load condition above a predeterminedvalue will fire uni-junction transistor 225 thereby applying signal 250to gate terminal 326 of silicon controlled rectifier 325. 'Ihe singlephase sensing arrangement 200 may be modified for multiphase operationby the addition of similar .arrangements (not shown) of members 202,206, 208, 210 and 212 -for the respective phases, connected to terminalsX, Y.

Isolating capacitors 130, 230 interconnect the gating output signals150, 250 of the uni-junction transistor elements 125, 225 to common gateterminal 326-, in a manner permitting independent opera-tion thereof.Thus, silicon controlled rectifier 325 will be switched responsive toeither time delayed heating of thermistor 10'4, or a short circuitoutput from current transformer -2.

I have obtained satisfactory operation with the following componentvalues being employed in the circuit 6 of FIGURE 2. It is to beunderstood that these values are merely representative componentparameters and are not intended to limit the scope of the invention.

125, ZZS-Uni-junction transistor-2N489- 325-Silicon controlledrectifier- 2Nl77lA. 26, 28-Diode--1N321l 116, 206, 208, 216, 218,334-Diode-1N1693. 36-Zener diode-INZSlSB.

104-50K at 25 C.

R106-Resistor--2K ohms pot.

It is therefore seen that my invention provides an improved arrangementlfor electrically operating a circuit breaker trip mechanism in a mannerpermitting increased ease of adjustment and calibrating accuracy overthe mechanical trip arrangements of the prior art.

Although I have described preferred embodiments of my novel invention,many variations and modifications will now be obvious to those skilledin the art, and I prefer therefore to be limited not by the specificdisclosure herein but only by the appended claims.

The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:

1. A circuit protective arrangement including the combination of: acircuit breaker having a heater element circuit connected to the circuitto be protected, such that said heater element develops heat in a manneroperatively related to circuit current, a pair of separable contacts,and an electromagnetically actuated fault responsive means for trippingsaid circuit breaker, such that said contacts separate for interruptingthe circuit current; and an overload sensing device, said overloadsensing device comprising a thermally responsive means in heat transferrelationship with said heater element; the electrical characteristics ofsaid thermally responsive means predeterminedly varying responsive tothe temperature thereof; said thermally responsive means being circuitconnected in a first circuit; a first switch means having a first andsecond operating condition; said first circuit presenting a first signalto said first switch means; the magnitude of said first signal varyingresponsive to the electrical characteristic variation of said thermallyresponsive means; a predetermined magnitude of said first signaleffecting switching of said first switch means between said first andsecond operating conditions; said predetermined mag.- nitudecorresponding to the temperature of said thermally responsive meansexceeding a predetermined magnitude; the switching of said first switchmeans providing a second signal to actuate said electromagneticallyactuated means and trip said circuit breaker in accordance with thepredetermined temperature of said thermally responsive means.

2. A circuit protective arrangement including the combination of: acircuit breaker having a heater element circuit connected to the circuitto be protected, such that said heater element develops heat in a manneroperatively related to circuit current, a pair of separable contacts,and an electromagnetically actuated fault responsive means for trippingsaid circuit breaker, such that said contacts separate for interruptingthe circuit current; and an overload sensing device, said overloadsensing device comprising a thermally responsive means in heat transferrelationship with a heater element; the electrical characteristics ofsaid thermally responsive means predeterminedly varying responsive tothe temperature thereof; said thermally responsive means being circuitconnected in a first circuit; a first switch means having a first andsecond operating condition; said first circuit presenting afirst signalto said first switch means; the magnitude of said first signal varyingresponsive to the electrical characteristic variation of said thermallyresponsive means; a predetermined magnitude of said first signaleffecting switching of said first switch means between said first andsecond operating conditions; said predetermined magni- 7 tudecorresponding to the temperatue of said thermally responsive meansexceeding a predetermined magnitude; the switching of said first switchmeans providing a second signal; a second switch means having a firstand second operating condition; said second signal being presented tosaid second switch means, and switching said second switch means betweenits first and second operating conditions; the switching of said secondswitch means between said first and second operating conditionsactuating said electromagnetically actuated means for tripping saidcircuit breaker in accordance with the predetermined temperature of saidthermally responsive means.

3. A circuit protective arrangement as set forth in claim 1, wherein:said first circuit includes an adjustable circuit element; theadjustment of said circuit element selectively varying the relationshipbetween said predetermined magnitude of said first signal and thetemperature of said thermally responsive means.

4. A circuit protective arrangement as set forth in claim 1, wherein:said thermally responsive means comprises a thermistor.

5. A circuit protective arrangement as set forth in claim 3, saidthermally responsive means comprising a thermistor; said adjustablecircuit element comprising a variable resistor in voltage dividerrelationship with said thermistor; the magnitude of said first signalrelated to the resistance relationship of said thermistor and variableresistor.

6. A circuit protective arrangement as set forth in claim 1, said firstswitch means being a solid state electronically operable element.

7. A circuit protective arrangement as set forth in claim 1, said firstswitch means being a uni-junction transistor; said first and secondconducting states corresponding to reverse and forward biasingrespectively of said uni-junction transistor.

8. An overloading sensing device for a circuit breaker, said overloadsensing device comprising a thermally responsive means in heat transferrelationship with a heater element; the electrical characteristics ofsaid thermally responsive means predeterminedly varying responsive tothe temperature thereof; said thermally responsive means being circuitconnected in a first circuit; a first switch means having a first andsecond operating condition; said first circuit presenting a first signalto said first switch means; the magnitude of said first signal varyingresponsive to the electrical characteristic variation of said thermallyresponsive means; a predetermined magnitude of said first signaleffecting switching of saidfirst switch means between said first andsecond operating conditions; said predetermined magnitu-de correspondingto the temperature of said thermally responsive means exceeding apredetermined magnitude; the switching of said first switch meansproviding a second signal; a second switch means having a first andsecond operating condition; said second signal being presented to saidsecond switch means, and switching said second switch means between itsfirst and second operating conditions; the switching of said secondswitch means between said first and second operating conditionselectrically tripping a circuit breaker in accordance with thepredetermined temperature of said thermally responsive means; said firstand second switch means being solid-state electronically operablecomponents; said first switch means being a uni-junction transistor;said first and second conducting states corresponding to reverse andforward biasing respectively of said uni-junction transistor; saidsecond switch means being a silicon controlled rectifier; said secondsignal being a gate input to said silicon controlled rectifier.

9. An overload sensing device for a circuit breaker, said overloadsensing device 4comprising a thermally responsive means in heat transferrelationship with a heater element; the electrical characteristics ofsaid thermally responsive means predeterminedly varying responsive tothe temperature thereof; said thermally responsive means being circuitconnected in a first circuit; a first switch means having a first andsecond operating condition; said first circuit presenting a first signalto said rst switch means; the magnitude of said first signal varyingresponsive to the electrical characteristic variation ofV said thermallyresponsive means; a predetermined magnitude of said first signaleffecting switching of said first switch means between said first andsecond operating conditions; said predetermined magnitude correspondingto the temperature of said thermally responsive means exceeding apredetermined magnitude; the switching of said first switch meansproviding a second signal; a second switch means having a first andsecond operating condition; said second signal being presented to saidsecond switch means, and switching said second switch means between itsfirst and second operating conditions; a third switch means inelectrical circuit relationship with an instantaneous load sensingmeans; a third signal being presented to said third switching meansproportionally responsive to instantaneous load; said third switch meanshaving a first and second operating con-dition; a predeterminedmagnitude of said third signal effecting switching of said second switchmeans between said rst and second operating conditions; saidpredetermined magnitude of said third signal corresponding toinstantaneous load exceeding a predetermined magnitude; the switching ofsaid third switch means providing a fourth signal; said fourth signalbeing presented to said second switch means and switching said secondswitch means between its first and second operating conditions; theswitching of said second switch means between said first and secondoperating conditions electrically tripping a circuit breaker inaccordance with both predetermined temperature of said thermallyresponsive element corresponding to switching of said switch means, andpredetermined instantaneous load corresponding to switching of saidthird switch means.

10. An overload sensing device as set forth in claim 9, furtherincluding isolating circuit-ry means for i-ndependently presented saidsecondV and fourth signals to said second switch means.

11. An overload sensing device as set forth in claim 9, said first,second and third switch means being solid-state electronically operableelements.

12. An overload sensing device `as set forth in claim 10; said first andthird switch means being uni-junction transistors; said first and secondoperating conditions corresponding to reverse and forward biasingrespectively of said uni-junction transistors; said secondy switch meansbeing a silicon controlled rectifier; said second `and fourth signalsbeing gate inputsl thereto; said isolating circuitry means being circuitconnected intermediate the output terminals of said uni-junctiontransistors, and the gate terminal of said silicon controlled rectifier.

13. In combination with a' circuit breaker having a heater element,instantaneous load sensing means an-d relay trip means; a solid statetripping means comprising a first, second and third switching means;each of said switching means having a blocking and conducting state; acircuit arrangement including transducer means in heat transferrelationship With said heater element for applying a signal to saidfirst switch means responsive to the temperature of said heater element;said first switch means being switched between its blocking andconducting states corresponding to said heater element being at apredetermined temperature; a circuit arrangement operatively connectedto said instantaneous load sensing means for applying a signal to saidthird switch means responsive to instantaneous load; said third switchmeans being switched between its blocking and conducting statescorresponding to instantaneous load being above a predeterminedmagnitude; the switching of said first or third switch means presentinga switching signal to said second switch means; said last-mentionedswitching signal switching said second switch means between its blockingand conducting states to elect operations of said relay trip means.

14. The combination as set forth in claim 13, wherein: said relay tripmeans is an undervoltage coil parallel circuit connected across saidsecond switching means, the switching of said second switch meanseiecting a short circuit across said undervoltage coil.

15. The combination of an instantaneous load sensing means and relaytrip means with a circuit breaker having a heater element, as set forthin claim 13; said circuit arrangement of said transducer means furtherincluding an Iadjustable circuit element in voltage divider relationshipwith said transducer means; the magnitude of the signal applied to saidrst switch means related to the resistance relationship of saidtransducer means and 4adjustable circuit element, whereby the adjustmentof said adjustable circuit element selectively varies the temperature ofysaid heater element corresponding to switching of said rst switchmeans.

10 16. The combination of an instantaneous load sensing means and relaytrip means with a circuit breaker having a heater element, as set forthin claim; further including isolation circuitry means `for independentlypresenting the switching signals of said irst and third switching meansto said second switching means.

References Cited UNITED STATES PATENTS 3,032,169() 5/ 1962 Elliot 317-13X 3,132,287 5/1964 Yarbrough. 3,209,206 9/ 1965 Courtin 317--13 X MILTONO. HIRSHFIELD, Primmy Examiner.

I D. TRAMMELL, Assistant Examiner.

1. A CIRCUIT PROTECTIVE ARRANGEMENT INCLUDING THE COMBINATION OF: ACIRCUIT BREAKER HAVING A HEATER ELEMENT CIRCUIT CONNECTED TO THE CIRCUITTO BE PROTECTED, SUCH THAT SAID HEATER ELEMENT DEVELOPS HEAT IN A MANNEROPERATIVELY RELATED TO CIRCUIT CURRENT, A PAIR OF SEPARABLE CONTACTS,AND AN ELECTROMAGNETICALLY ACTUATED FAULT RESPONSIVE MEANS FOR TRIPPINGSAID CIRCUIT BREAKER, SUCH THAT SAID CONTACTS SEPARATE FOR INTERRUPTINGTHE CIRCUIT CURRENT; AND AN OVERLOAD SENSING DEVICE, SAID OVERLOADSENSING DEVICE COMPRISING A THERMALLY RESPONSIVE MEANS IN HEAT TRANSFERRELATIONSHIP WITH SAID HEATER ELEMENT; THE ELECTRICAL CHARACTERISTICS OFSAID THERMALLY RESPONSIVE MEANS PREDETERMINEDLY VARYING RESPONSIVE TOTHE TEMPERATURE THEREOF; SAID THERMALLY RESPONSIVE MEANS BEING CIRCUITCONNECTED IN A FIRST CIRCUIT; A FIRST SWITCH MEANS HAVING A FIRST ANDSECOND OPERATING CONDITION; SAID FIRST CIRCUIT PRESENTING A FIRST SIGNALTO SAID FIRST SWITCH MEANS; THE MAGNITUDE OF SAID FIRST SIGNAL VARYINGRESPONSIVE TO THE ELECTRICAL CHARACTERISTIC VARIATION OF SAID THERMALLYRESPONSIVE MEANS; A PREDETERMINED MAGNITUDE OF SAID FIRST SIGNALEFFECTING SWITCHING OF SAID FIRST SWITCH MEANS BETWEEN SAID FIRST ANDSECOND OPERATING CONDITIONS; SAID PREDETERMINED MAGNITUDE CORRESPONDINGTO THE TEMPERATURE OF SAID THERMALLY RESPONSIVE MEANS EXCEEDING APREDETERMINED MAGNITUDE; THE SWITCHING OF SAID FIRST SWITCH MEANSPROVIDING A SECOND SIGNAL TO ACTUATE SAID ELECTROMAGNETICALLY ACTUATEDMEANS AND TRIP SAID CIRCUIT BREAKER IN ACCORDANCE WITH THE PREDETERMINEDTEMPERATURE OF SAID THERMALLY RESPONSIVE MEANS.